978-1-4673-0311-8/12/$31.00 ©2012 IEEE

A Web-based Service Backend for Control and

Monitoring of Solar Parks

Apostolos Meliones and Angeliki Nouvaki

Department of Digital Systems, University of Piraeus, Greece

meliones@unipi.gr, nouvaki@gmail.com

Abstract—The architecture of a control system can be de-

signed vertically with the distinction between functional levels.

We adopt this layered approach for the design and

implementation of a network-based control and monitoring

application. The main services provided to the user include

control, monitoring, notification, reporting and data export. Our

proposed system consists of a front-end for system automation

and a service backend which undertakes to collect, store and

manage data from all remote installations. We have successfully

applied it to solar parks; nevertheless it can be easily migrated to

other remote automation applications and renewable energy

generation installations.

Keywords—automation, control and monitoring system, solar

park, photovoltaic plant, PV power plant, renewable energy

generation, service backend

I. INTRODUCTION

Due to the intensifying climate challenge, green growth is expected to be a very important issue in the next decades. Green growth strategies can help economies and societies become more resilient as they work to meet demands for food production, transport, housing, energy and water.

A photovoltaic system (PV System) is a system that consists of one or more solar panels which converts sunlight into electric power. In recent years, photovoltaic systems present a rapid growth. Every year thousands of installations and thus MWp (megawatt peak) are completed. Due to high energy saving, energy efficiency and the existence of optimal conditions, such as subsidies, important capital is invested in research and construction of PV technology.

Such systems, however, cannot work properly if operations are not automated. For this reason, the need of an automated and integrated system which controls and monitor solar parks is imperative. The development of automated applications in the energy sector is significant, not only in industry but also in our daily lives. End-users increasingly demand products that use less energy, something which conflicts with the growing demand for expanded functionality. Control and automation applications are developing solutions to reduce costs in time, money and effort of procedures aiming to improve the quality of either a service or a product.

In this paper we present an implemented network-based solar park application for controlling and monitoring the input and output data of the equipment aiming at the recognition of low performance, alarm detection, operation failures, security through cameras, access control, collection and recording of statistical data and provisioning of reports which are stored

locally or sent as emails. The paper describes a three level architecture for the application, available equipment, signals, alarms, and implemented services. The automation system architecture is presented in Section 2. The main control and automation services provided are described in Section 3. A description of the solar park installation, the monitored signals and implemented alarms follow in Sections 4, 5 and 6. Section 7 illustrates the implemented services via the application screenshots, while Sections 8 and 9 present the technology we used to build our application and the database schema.

II. AUTOMATION SYSTEM ARCHITECTURE

The architecture of a control system can be designed vertically, with the distinction between functional levels [1]. Our presented architecture shown in Fig. 1 consists of three levels. Each of them represents an aspect of system functionality, presenting the incarnation of the automation pyramid for a solar park automation system [2]. The bottom level or the level of devices interacts with the real world and is constituted by the basic equipment of an automation system. This includes sensors or other measure means according to the field of application, actuators, data converters (digital to analog data and reversely) and routers [3]. In addition, PLCs or PACs are employing all necessary interfacing and storage functionality deployed at each installation separately. After the collection of data through measurement, counting, metering, they are transformed into a representation suitable for transmission and processing.

The role of field controllers is very important. They recog-nize failures and generate alarms derived either directly from signals I/O or complex algebraic, combinatorial or sequential functions and processes. Subsequently, they categorize and prioritize, store, generate alerts and reports and communicate the status of the installation to the management centers.

The next level is the Data Center which communicates through a public and/or private network with the bottom level. It is also known as automation level. Its role is to monitor and control all installations in a centralized manner. The main part of the Data Center is the Service Backend, which undertakes to collect, store and manage data from all remote installations. The Service Backend has the following responsibilities:

To collect, store and manage data from a variety of remote sites.

To transform data and prepare values for vertical access from the management level.

To store and manage information regarding the users

978-1-4673-0311-8/12/$31.00 ©2012 IEEE 229

Internet Wireless

network (2G/3G)

Private/public

network

Communication

Module

Database Reporting Module

Application Logic Module

Notification Module

Service Back-End

Internet gateway SMS gateway

Data Center

Sensors, actuators,

other equipment

Sensors, actuators,

other equipment

Sensors, actuators,

other equipment

LAN

Router

PLC PLC PLC

…

accessing the service.

To allow the users to configure the service back-end itself or the field-deployed controllers.

To allow the users to carry out control functions on the installation.

The service back-end also incorporates the following modules:

Communication Adapter Module (CAM), which undertakes the task of acquiring data from the remote installation and feeding them to the service back-end as well as conveying data from the service back-end to the remote site. Multiple CAMs are supported because of the variety of third-party controllers.

Application Logic Module (ALM), which calculates the derived parameters. This module can be extended with new calculation functions.

Notification Server Module (NSM), which undertakes the notification of the users via SMS or e-mail.

Reporting Server Module (RSM), which handles the generation and delivery of reports. All necessary data are stored in a database.

Figure 1. Automation System Architecture

The top level is the management level. At this level, information is accessible from the entire system. A unified interface is presented to the operator and end-users for manual

intervention in the system. It provides vertical access to the values of the automation level, including the modification of parameters such as timetables. Alarms are generated for exceptional situations such as technical errors or critical conditions. The long-term storage of historical data with the ability to give commands for reports and statistics is also part of this level.

III. MAIN CONTROL AND AUTOMATION SERVICES

Control and automation applications provide a number of services. Some of them have their own module in the previously described architecture. The main services are:

A. Monitoring Service

The main functionality of automation systems is to provide consistently and accurately useful information of the system to the user. Installation data is collected by the equipment in the field level, which is also responsible for calculating primary parameters. Primary parameters are average values of real time data across specific time intervals. Daily, monthly and annual averages are produced, as well as maximum and minimum values of primary parameters. These procedures are executed in low level in order to reduce the amount of data that passes to the service back-end and reduce demands on bandwidth. Primary parameters are being transferred to the service back-end, where they are used for the calculation of derived parameters. By calculating derived parameters in the service back end we reduce demands on bandwidth and storing at the remote site. Furthermore, real time data as well as raw data are being stored in a system database.

User Interface

Back-end Service

Commu cation Module

Database

Monitoring &

Control System

Reporting Module

Application logic module

Notification Module

Figure 2. Monitoring Service Architecture

B. Control Service

The user can control his installation, equipment and its functions via an automation system. Set values are fixed until they are changed during the course of installation or by the user through the control front-end application. Figure 3 depicts the flow of data in the Control Service. The user commands pass to the service back-end which initiates all related operations (executing command on the field equipment, access to database, returning notification for given command).

C. Notification Service

The Notification service informs the user when important events take place. Notification is through emails (internet

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gateway) or SMS (sms gateway). Notifications are created when an equipment node comes to a state which produces signals which bypass or underpass specified thresholds and remain for a long period of time. The user can set the limit values for each signal, as well as indicate the means by which he wants to be notified.

User Interface

Back-end Service

Communication Modul

Database

Monitoring &

Control System

Figure 3. Control Service Architecture

D. Reporting Service

A very important function is the automatic generation of reports. This service uses the Web and delivers reports to e-mails or mobile phones of users who have subscribed to the service. These reports include information on the operation, maintenance and financial situation of the entire system or specific facilities. The timing and scope is determined by the administrator and the choices of the user.

E. Data Export Service

The Data export service allows the user to store locally data that is stored in the application. The source of data is primary or derived parameters from any entity that is monitored. The extracted data is in xml file format or graphic representations. Regarding the time frequency of data extraction and export, these are determined by the user.

IV. SOLAR PARK INSTALLATION

The operation of solar parks is based on the photovoltaic effect. The photovoltaic effect is the basic physical process in which a photovoltaic cell converts sunlight into electricity. A solar park consists of solar panels. Each panel is a set of solar cells and numerous other protective and functional coatings installed onto an aluminum frame as a means of support. They are connected in series until we get the desired effect on current output and in parallel until they reach the desired voltage. As a result, solar panel connection is very important during installation.

A very important equipment of solar parks is battery. The battery stores solar energy to provide electricity during periods of absence of sun, such as nights and cloudy days. It must be able to be discharged and recharged. Rechargeable batteries are a bit more expensive than the reserve batteries. Without batteries, a PVC system can provide electricity only with sunlight. Along with the battery there should be a charge controller. Its role is to provide electricity to the batteries from

the solar panel in a way that prevents the solar panels from overloading the battery.

Lastly, solar parks have power inverters. Solar panels generate electricity to DC (Direct Current) power. Some devices are powered directly from the panel. However, most devices use high voltage AC (Alternating Current) power. The power converter performs this procedure. Converts low voltage DC from the battery to high voltage alternating current AC that is required by most household appliances.

V. SIGNALS

Every control and monitoring application controls a number of signals using the appropriate sensing elements. Each transducer gives some type of digital (binary or discrete) or analog (continuous) signal, or more complex signals in the form of a protocol, e.g. serial. The type of signal depends on the observed physical size and the transducer used [4].

Controllers inside an automation system can control and monitor digital, analog and serial data from sensors, actuators and devices. Our implementation for controlling and monitoring the power of a solar park uses the following signals:

1) Power Signal: It is an analog signal. This paper suggests

an implementation which uses AC power. This consists of three

components:

a) Real/Active Power: The amount of energy consumed;

measured in Watts.

b) Apparent Power: The amount of energy delivered

from a source to a charger. It must always be greater than that

needed from a device to work; measured in Volt-Amperes (va)

c) Reactive Power: The amount of energy that returns to

the source in each cycle because of the stored energy;

measured in reactive Volt-Amperes (var).

2) Temperature Signal: It is an analog signal used for

checking the proper functioning of the system in extreme

environmental conditions. Sensors measure the temperature of

the installation, both internal (module temperature) and

external (air temperature). The ideal temperature for solar

power plants is 25°C (77°F). When temperature increases, the

current increases slightly, while the voltage is reduced too

quickly. This results in a lower overall energy output. A

general rule is that the performance of a cell drops by 0.5% per

1°C above the 25°C.

3) Humidity Signal: It is an analog signal. A sensor

measures the humidity of the environment which is used for

monitoring the proper functioning of the system especially

when signal’s value goes over/under a specified threshold.

4) Wind Signal: It is an analog signal used for the same

reason as humidity signal.

The system has been tested with real data. These were collected during a period of one year from a real PV installation which collected and monitored the above signals. Figure 4 depicts the annual power produced in our monitored park in relation with air temperature and humidity in the park area. The maximum performance is in May, while the minimum appears in mid-December. The performance increases during the

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summer months due to the favorable climate. Apparently, performance change is proportional to temperature change. The minimum performance has been recorded during the day of the year with the minimum air temperature. This synchronization between the two plots does not happen for maximum performance. That means that temperature is not the only factor affecting the performance of a solar park. The total annual power produced by the solar park reached 1.82 MW.

Figure 4. Annual Produced Power vs Air Temperature and Humidity

Figure 5 depicts the power and temperature values across a winter and a summer day. Recordings on 12 Aug 2010 - a hot summer day with an outside temperature of an average 30°C - show a peak of 900KWatts. Recordings on 12 Dec 2010 - a cold winter day with an outside temperature of a mere 5°C - show that it is easily and permanently possible to achieve the goal of 70°C and a thermal energy of almost 1,5 MWh in the storage units: a very good result for almost the shortest and coldest day of the year. In both cases, it is observed that the energy produced (apparent power) by the park is entirely consumed (active power).

Figure 5. AC Power vs Temperature (12 Aug and 12 Dec)

VI. ALARMS

Alarms are binary signals which depend on one or more signals and activated by the changes of their values and situation. In the case of binary digital signals the alarm is activated by the state change of the signal. In case of analog

signals, activation of the alarm takes place when the signal value exceeds or goes under a threshold. Our solar park control and monitoring application implements the following alarms:

1) Temperature Alarm (air & module alarm): The

manufacturer or user sets minimum and maximum values for

air and module temperature in order to control the environment

of a solar plant system in extreme weather conditions, such as

frosts or large heat waves. Temperature alarms are created

when values exceed these thresholds. The ideal temperature for

solar power plants is 25° C (77 ° F).

2) Power Loss Alarm: This alarm is activated by a

combination of factors. Some of them are long periods of

shadow, battery performance and temperature. As mentioned

before (see temperature signal), the increase of temperature

results in a lower overall energy output.

3) Battery Alarm: The amount of kWatts per hour that a

solar panel can produce with battery life depends on the

temperature and on sun exposure. The manufacturer or user

sets minimum/maximum thresholds and when temperature’s

value is out of this range for a long period then a battery alarm

is created. The performance of a battery is reduced below a

threshold and battery stops charging over a temperature limit.

4) Mold Alarm: It is created with the logic of specified

thresholds for valid values, as described in temperature alarm.

5) Security Alarm: Allows a user to control the installation

of any disasters/violations, through optical fibers applied to

solar panels or used for installation’s fence.

VII. IMPLEMENTED SERVICES

Our proposed automation system for controlling and monitoring solar parks implements a group of services, as these described at the previous sections.

A. Surveillance

Every installation has cameras in specified areas according to pairs of coordinates (south, north, west and east). The user can monitor with live video every plant by specifying the direction s/he wishes, and even handling PTZ (pan, tilt, zoom) functionality if available (see Fig. 6).

Figure 6. Surveillance Service

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B. Performance

This service is very important because it gives the opportunity to the user to monitor very quickly the total performance of his/her park. Not only can see the current performance, but also can give specific time of period to check. Through this service the user can compare the performance between days, months, years or certain time intervals and draw conclusions (see Fig. 7).

Figure 7. Performance Service

C. Data Export

Through the data export page the user can choose one sig-nal or group of signals, the period of time that s/he is interested in their values and the unit (X axis) of representation (days, months, years). The results are being represented by graphical plots. The user further has the opportunity to store these values locally in an xml file or send them via email (Fig. 8).

Figure 8. Data Export Service

D. Alarm

Through the alarm service the user can check the thresholds and priority for each supported signal. If s/he is the administrator, s/he can further set these values. Alarm notifications are generated based on these values. Two specific alarms - battery and performance - are generated automatically

by functions inside the service back-end of our architecture. For these two alarms the user can only define their threshold in percentage. For example, a notification is being generated when battery reaches 15% of maximum performance. In addition, this service displays a graphical representation for a selected alarm according to a specified period allowing the user to easily compare values in relation with the max threshold range depicted with a blue area (see Fig. 9).

Figure 9. Alarm Service

E. Notification

The user chooses from his/her profile page which communication channel will be used according to the type of notification and alarm. Three channels are provided by the system: (i) An sms channel to receive notifications in a mobile phone. (ii) An email channel, where the user receives notifications through the email s/he filled in the registration form. (iii) Inbox, which is implemented through a web page of our system called ‘Notifications’ and uses system’s local resources. This page is accessible through the system environment of a logged user. It is easy to use and very practical because all the information is gathered.

VIII. TECHNOLOGY

The solar park system is implemented in Visual Studio 2010 with Framework .NET 4. The programming language used is the object-oriented C# language v4.0. The application consists of three projects. The first project is a web site where all .aspx pages are implemented. The aspx pages comprise the user interface. Each page has its own file code behind where functions of the Application Logic Module are implemented.

The second one is a Class Library project named DBTier. This project provides the Database Access Layer of the solution and establishes the connection to the system database. It contains files for each database table and communicates with the Application Logic, Reporting and Notification Modules, by carrying out insertions and updates or sending them feedback via query results for every function they operate.

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Figure 10. Notification Inbox Service

The last project is a WCF Service Library project, which implements the Web Service of our application. WCF is an API in the .NET Framework for building connected, service-oriented applications. Our application has implemented a Mail Sender Web Service, which sends notification emails when an alarm is created. For this purpose we used the System.Net.Mail library of the .NET framework. This library contains classes in order to send email to a designated SMTP server for delivery. Notification mails include reports as attachments.

The Visual Studio Integrated Development Environment in-cludes the creation of WCF Service Library for Web Services implementation. The WCF Service Library is a compiled component that can be expanded as a Web Service or Windows Service or even as part of a custom application hosting. When the debugger starts, the WCF Service Host (WcfSvcHost.exe) implements the hosting of the Web Services projects. Then it opens the WCF Test Client (WcfTestClient.exe) and displays a list of services endpoints that have been defined in configura-tion file. Therefore, the Wcf Test Client tool helps experience the implementation of Web Services and gives the opportunity to users to test their Web Services without having to execute the web application, and to implement the WcfSvcHost hosting without the need for setting up of Web Services using the IIS (Internet Information Services) Microsoft web server.

The Graphical representations of our application were implemented using Charting Controls which are embedded in Framework 4.0. With these tools we can specify the type of graph, the format of the source of data and the filter of data.

IX. DATABASE SCHEMA

The database on the service back-end was implemented in Microsoft SQL Server 2008 and consists of twelve tables:

1) Park: contains all registered parks.

2) Panel: contains all registered panels, each connected to

a single park.

3) User: contains data for registered users who have

access to a solar park.

4) PanelData: stores signals’ values. These values derive

from the CAM, which acquires data from the remote

installation and calculates values’ average of a time span of 30

seconds. The values stored are: battery, humidity, wind speed,

air and module temperature, as well as active, reactive and

apparent power.

5) ContactMedia: contains all available contact media

(email-sms-inbox).

6) UserContactMedia: registers the media by which the

user is notified for each alarm.

7) Notification: saves all inbox notifications which are

automatically created by the system.

8) Priorities: contains all available priorities for system’s

notification (low-medium-high).

9) ParkUser: connects users with parks defining a role for

each relation.

10) Roles: contains the supported user roles of the system.

11) Thresholds: provide minimum and maximum value for

all measured signals.

12) ParkThreshold: relates each park with its active

thresholds.

X. CONCLUSIONS

We have developed a flexible modular network-based application which supports all key services for automatic re-mote control and management of photovoltaic parks. Our implemented application fully achieved the systems’ objectives meeting successfully the initial user requirements. We have presented in detail a three-tier layered architecture and the according design and implementation of a modular service back-end system including database, notification, reporting and arithmetic logic system components. The presented integrated system allows the easy and convenient monitoring of a real operational environment and helps the executives to administer their solar parks. The Service Backend can provide online customized monitoring of signal groups and system status, energy production and environmental benefits in real-time. We have tested our system with real data collected and monitored during a period of one year from a 2MWp PV plant installation. The monitored panel data included several signals: active, reactive and apparent power, air and module temperature, battery, humidity and wind speed.

The presented system is a successful paradigm of automation technology and integration of front- and back-end system components which can be easily migrated to solar buildings and even entire solar cities for solar energy exploitation, as well as other types of renewable energy generation installations and remote automation applications.

REFERENCES

[1] Passino M. Kevin, “Biomimicry for optimization, control, and automation”, Part I, pp. 7-55 , DOI: 10.1007/1-84628-069-9_1 , 2005

[2] Kastner W., Neugschwandtner G., Soucek S., Νewman H.M, “Communication Systems For Building Automation And Control”, Proceedings of The IEEE, Vol.93, No.6, 2005

[3] Dorf R.C., Bishop R.H., “Modern Control Systems”, 10th edition, ISBN-10: 0131457330, 2004

[4] Brown, Jr. et al., “Energy Management And Home Automation System”, United States Patents, 1996

[5] Shimon Y., Nof (Ed.) , “Springer Handbook of Automation”, ISBN 978-3-540-78830-0, 2009

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